Safety imperatives require that, in certain cases of use such as are encountered in the nuclear, oil, aeronautic, space, naval, and chemical industries, those circuits which are conveying energy or transmitting monitoring or control signals must withstand the high temperatures which are due, for example, to fires, or, for electrical circuits, to abnormal rises in the intensity of the electric current which passes through them, for a sufficient period of time so as to enable personnel to be evacuated and equipment to be saved. In the case of short circuits or overintensity, it is also desired that the considerable increase in temperature of the conductor, or even the fusion thereof, does not provoke a fire by combustion of the coating.
Wholly or partly in response to these reuqirements, ceramic coatings, some of which are associated with glass fibers, have already been used. Furthermore, there are coatings, particularly for electrical cables, which result from the association of fabrics of glass fibers and metallic oxides.
The electrical cables intended for these uses exist in both reinforced and non-reinforced forms, depending on whether they are manufactured in a form which is protected from the risks of mechanical deterioration by a rigid metal coating, or whether they must be introduced into metal tubes during their assembly so as to insure the protection thereof.
The reinforced cables are made up of one or more conductors insulated by a substance with a low carbon content and protected by a cylindrical shell obtained by winding a metal ribbon, or by a metal tube. The non-reinforced cables are also made up of conductors insulated by substances with a low carbon content, but they are sheathed by a complex of glass fiber and sliliconed rubber, or any other non-combustible material.
Among these insulating coatings which present a considerable resistance to high temperatures, reference can be made to those described in the following patents:
French Pat. No. 2,381,377, which describes a coating which comprises a metal tube packed with magnesia; French Pat. No. 2,257,555, which describes a coating composed of a layer of inorganic non-combustible fiber which constitutes a heat insulator, and a layer of halogen bonded to non-combustible fiber by a resin; French Pat. No. 2,462,771, which describes a coating comprised of baked insulating mineral matter impregnated with silicon oil and protected by a metal sheath; French Pat. No. 2,482,769, which describes a heat-resistant, flexible, refractory, insulating coating composed of a porous basic material and a refractory coating capable of melting with the basic porous material at high temperature; French Pat. No. 2,206,563, which describes a high temperature insulation composed of a borosilicate glass and silica, which is melted at high temperature to form a mass having a viscosity greater than the viscosity of the glass and the silica at the same temperature; French Pat. No. 2,360,530, which describes a fritted, vitrifiable body based on glass and quartz; U.S. Pat. No. 3,602,636, which describes a coating comprising a helical covering with a glass mterial, with open weave, bearing a coating of synthetic rubber which is flame-resistant, and is protected by a sheath of polyvinylchloride; U.S. Pat. No. 3,632,412, which describes a self-adhesive tape comprising an interpolymer lined with a glass cloth; U.S. Pat. No. 3,013,902, which describes materials coated with colloidal alumina; U.S. Pat. No. 3,095,336, which describes the preparation of laminated ceramic products with glass cloth; and European Pat. No. 80.107217.4, which describes an insulating coating of ceramics comprising a polyimide and mica.
In an attempt to respond to the particularly severe requirements of certain uses, particularly those on oil platforms, in the mining industries and in nuclear power stations, requirements have arisen for the installation of explosion- and fire-proof conduits. These types of installations, although limiting the effects of self-propagation and the release of fumes, have presented the drawback of confining the heat to inside the conduit, which has the effect of destroying the insulator, and of creating ruptures in the cables and short circuits therein.
In addition to the above, although insuring the protection of the electrical cables and maintaining the insulation indispensable for a predetermined duration, the majority of the above-mentioned prior coatings, do not allow the installations to be subsequently re-used without first having all of the wiring replaced. These destructive results are further accentuated when extinction means such as the projection of sea water or immersion are used.
All of this is also apart from the added problems faced by those fluid conduits which supply compressed gas or hydraulic liquid to remote equipment which may be vital to maintaining operations. The supple pipes which are generally used in these cases are only very imperfectly protected from direct heat, and they are not designed to resist a considerable rise in temperature for a sufficiently long time.